RFC: Enable CONFIG_USB_GADGET_DUMMY_HCD ?

[TJ]

We currently have the USB Gadget interface enabled with the peripheral
controller CONFIG_USB_GADGET_NET2280. We also have
CONFIG_USB_FILE_STORAGE which gives us the g_file_storage module.

So far I haven't found anything in the changelog or git commits to
indicate if CONFIG_USB_GADGET_NET2280 is an inherited default or choice
based on some scenario we need.

If we were to enable CONFIG_USB_GADGET_DUMMY_HCD instead of
CONFIG_USB_GADGET_NET2280 to give us the dummy-hcd module, we can
provide the ability for the user to create virtual USB mass-storage
devices on the host from a backing file-store.

This can be very useful especially for VMs such as KVM, VMWARE and so
forth because it presents a virtual USB mass storage device to the
guest.

This is a quite separate facility from using a loop mount for creating
block devices on a file backing-store.

For example:

modprobe dummy-hcd
modprobe g_file_storage file=/home/all/VirtualMachines/usb-storage.img

kvm ... -usb -usbdevice host:X.Y

E.g.
dd if=/dev/zero of=/home/all/VirtualMachines/usb-storage.img bs=1024
count=2048

nc -l -p 9999 >boot.log &

kvm -name Ubuntu-Jaunty-Desktop -serial tcp:127.0.0.1:9999 -monitor
stdio -m 512 -hda ubuntu-jaunty-desktop.qcow2 -boot c -k en-gb -net
nic,model=rtl8139,macaddr=56:44:45:30:31:32,vlan=0 -net
tap,ifname=tap0,script=no,vlan=0 -soundhw es1370 -usbdevice
host:0525:a4a5

[    5.536627] usb 1-2: new full speed USB device using uhci_hcd and address 2
[    6.181501] usb 1-2: configuration #1 chosen from 1 choice
[    6.299703] Initializing USB Mass Storage driver...
[    6.317778] scsi2 : SCSI emulation for USB Mass Storage devices
[    6.324037] usbcore: registered new interface driver usb-storage
[    6.339894] USB Mass Storage support registered.
[   11.356923] scsi 2:0:0:0: Direct-Access     Linux    File-Stor Gadget 0302 PQ: 0 ANSI: 2
[   11.436330] sd 2:0:0:0: [sdb] 4096 512-byte hardware sectors: (2.09 MB/2.00 MiB)